This invention relates to a method of operating a plasma addressed liquid crystal (PALC) device to reduce sputtering.
U.S. Pat. No. 5,077,553 discloses apparatus for addressing data storage elements. A practical implementation of the apparatus shown in U.S. Pat. No. 5,077,553 is illustrated schematically in FIG. 1 of the accompanying drawings.
The display panel shown in FIG. 1 comprises, in sequence from below, a polarizer 2, a channel member 4, a cover sheet 6 (commonly known as a microsheet), a layer 10 of electro-optic material, an array of parallel transparent data drive electrodes (only one of which, designated 12, can be seen in the view shown in FIG. 1), an upper substrate 14 carrying the data drive electrodes, and an upper polarizer 16. In the case of a color display panel, the panel includes color filters (not shown) between the layer 10 and the upper substrate 14. The panel may also include layers for improving viewing angle and for other purposes. The channel member 4 is typically made of glass and is formed with multiple parallel channels 20 in its upper main face. The channels 20, which are separated by ribs 22, are filled with an ionizable gas, such as helium. An anode 24 and a cathode 26 are provided in each of the channels 20. The channels 20 are orthogonal to the data drive electrodes and the region where a data drive electrode crosses a channel (when viewed perpendicularly to the panel) forms a discrete panel element 28. Each panel element can be considered to include elements of the layer 10 and the lower and upper polarizers 2 and 16. The region of the upper surface of the display panel that bounds the panel element constitutes a single pixel 30 of the display panel.
When the anode 24 in one of the channels is connected to a reference potential and a suitably more negative voltage is applied to the cathode 26 in that channel, the gas in the channel forms a plasma which provides a conductive path to the reference potential at the lower surface of the cover sheet 6. If a data drive electrode is at the reference potential, there is no significant electric field in the volume element of electro-optic material in the panel element at the crossing of the channel and the data drive electrode and the panel element is considered to be off, whereas if the data drive electrode is at a substantially different potential from the reference potential, there is a substantial electric field in that volume element of electro-optic material and the panel element is considered to be on.
It will be assumed in the following description, without intending to limit the scope of the claims, that the lower polarizer 2 is a linear polarizer and that its plane of polarization can be arbitrarily designated as being at 0xc2x0 relative to a reference plane, that the upper polarizer 16 is a linear polarizer having its plane of polarization at 90xc2x0, and that the electro-optic material rotates the plane of polarization of linearly polarized light passing therethrough by an angle which is a function of the electric field in the electro-optic material. When the panel element is off, the angle of rotation is 90xc2x0; and when the panel element is on, the angle of rotation is zero.
The panel is illuminated from the underside by an extended light source 34 which emits unpolarized white light. A rear glass diffuser 18 having a scattering surface may be positioned between the light source and the panel in order to provide uniform illumination of the panel. The light that enters a given panel element from the source is linearly polarized at 0xc2x0 by the lower polarizer 2 and passes sequentially through the channel member 4, the channel 20, the cover sheet 6, and the volume element of the electro-optic material toward the upper polarizer 16 and a viewer 32. If the panel element is off, the plane of polarization of linearly polarized light passing through the volume element of electro-optic material is rotated through 90xc2x0, and therefore the plane of polarization of light incident on the upper polarizer element is at 90xc2x0. The light is passed by the upper polarizer element and the pixel is illuminated. If, on the other hand, the panel element is on, the plane of polarization of the linearly polarized light is not changed on passing through the volume element of electro-optic material. The plane of polarization of light incident on the upper polarizer element is at 0xc2x0 and therefore the light is blocked by the upper polarizer element and the pixel is dark. If the electric field in the volume element of electro-optic material is intermediate the values associated with the panel element being off and on, light is passed by the upper polarizer element with an intensity which depends on the electric field, allowing a gray scale to be displayed.
In a practical implementation of the PALC display panel, the channel member 4 is etched back around the area in which the channels are formed in order to provide a plateau 36 in which the channels 20 are formed, and the cover sheet 6 is secured to the channel member by an endless frit bead 38 in a rabbet 40 extending around the periphery of the plateau. An upper substrate assembly, including the upper substrate 14 and the data drive electrodes 12 carried thereby, is attached to the channel member 4 by means of a glue bead 42.
The voltages that are applied to the cathode and the data drive electrodes typically vary in accordance with the waveforms shown in FIG. 2. The anode 24 (waveform A) is held at a reference potential level, which may be ground. To write data in a single line, the data drive electrodes (waveform B) are driven so that there is a voltage difference of up to about 80 volts between each data drive electrodes and the anode 24. The actual voltage to which a given data drive electrode is driven depends on the desired gray scale level of the pixel at the crossing of the data drive electrode and the channel. Generally, the polarity of the voltage applied to the data drive electrodes alternates on successive frames to eliminate DC offset effects in the liquid crystal. The cathode 26 (waveform C) is driven to a negative firing voltage Vf, which is typically in the range xe2x88x92150 to xe2x88x92500 volts in order to initiate a discharge in the channel, and is then held at a negative sustain voltage Vs, which is typically less negative than the firing voltage. Finally, the cathode returns to ground and the discharge is extinguished.
During operation of a PALC panel having the structure described with reference to FIG. 1, the cathode 26 is subject to sputtering by the positive ions of the plasma. Sputtering can degrade performance of the PALC panel because the particles that are sputtered from the cathode are deposited on the interior surface of the channel and on the underside of the cover sheet and may reduce the transmissivity of the panel and hence its contrast during operation.
It is believed that sputtering is most severe immediately after the discharge has been initiated but before the voltage difference between the channel electrodes has been reduced to the sustain voltage because the potential difference between the anode and the cathode is at a maximum and therefore the charged particles striking the cathode are at their most energetic.
A discharge that is initiated in an ionizable gas between two electrodes that are both exposed to the gas is known as a DC discharge. A discharge can be initiated in an ionizable gas even if at least one of the electrodes is electrically insulated from the ionizable gas. Such a discharge is known as an AC discharge.
Modeling suggests that when the cathode of a PALC panel is driven to the firing voltage, first of all a discharge occurs in the vicinity of the underside of the cover sheet, and that the DC discharge between the anode and cathode occurs only subsequently. It is believed that the discharge that occurs in the vicinity of the underside of the cover sheet is an AC discharge between the cathode and the data drive electrodes. Thus, if the channel electrodes were appropriately driven, it would be possible to induce an AC discharge where the underside of the cover sheet is the cathode surface.
In accordance with a first aspect of the present invention there is provided an improved method of operating a PALC panel which comprises a channel member defining at least one channel, an ionizable gas in the channel, channel electrodes exposed to the ionizable gas in the channel, a cover sheet over the channel, a data drive electrode in crossing relationship with the channel and isolated from the channel by the cover sheet, and a layer of electro-optic material between the data drive electrode and the cover sheet, the method comprising initiating an AC discharge in the channel by driving at least one of the channel electrodes to a positive voltage relative to the data drive electrode, and thereafter maintaining a sufficient voltage between the channel electrodes to sustain a DC discharge in the channel.
In accordance with a second aspect of the present invention there is provided an improved PALC panel of the kind comprising a channel member defining at least one channel, an ionizable gas in the channel, channel electrodes exposed to the ionizable gas in the channel, a cover sheet over the channel, a data drive electrode in crossing relationship with the channel and isolated from the channel by the cover sheet, and a layer of electro-optic material between the data drive electrode and the cover sheet, wherein the improvement resides in a layer of transparent electron-emissive material on the underside of the cover sheet.
In accordance with a third aspect of the present invention there is provided an improved PALC panel of the kind comprising a channel member defining at least one channel, an ionizable gas in the channel, channel electrodes exposed to the ionizable gas in the channel, a cover sheet over the channel, a data drive electrode in crossing relationship with the channel and isolated from the channel by the cover sheet, and a channel electrode driver which drives at least one of the channel electrodes to a voltage such as to initiate a discharge in the channel, wherein the improvement resides in that the channel electrode driver drives at least one of the channel electrodes to a positive voltage relative to the data drive electrode, said positive voltage being of sufficient magnitude to initiate an AC discharge in the channel, and thereafter maintains a sufficient voltage between the channel electrodes to sustain a DC discharge in the channel.
In accordance with a fourth aspect of the present invention there is provided a PALC panel comprising a channel member defining at least one channel, an ionizable gas in the channel, channel electrodes exposed to the ionizable gas in the channel, a cover sheet over the channel, a data drive electrode in crossing relationship with the channel and isolated from the channel by the cover sheet, a layer of electro-optic material between the data drive electrode and the cover sheet, and a coating of transparent electron-emissive material on an interior surface of the channel.
In accordance with a fifth aspect of the present invention there is provided an improved PALC panel of the kind comprising a channel member defining at least one channel, an ionizable gas in the channel, channel electrodes exposed to the ionizable gas in the channel, a cover sheet over the channel, a data drive electrode in crossing relationship with the channel and isolated from the channel by the cover sheet, and a layer of electro-optic material between the data drive electrode and the cover sheet, wherein the improvement resides in that one of the channel electrodes is composed of a metal busbar extending longitudinally of the channel and a strip of transparent conductive material projecting laterally of the metal busbar.